There are widely used incandescent light bulbs which produce light with a filament such as tungsten filament heated by an electric current flown through it. Incandescent light bulbs have various advantages, for example, (a) they are inexpensive, (b) they show superior color rendering properties, (c) they can be used with any operating voltage (they can work with either alternating current or direct current), (d) they can be lightened with a simple lighting implement, (e) they are used worldwide, and so forth. However, efficiency of incandescent light bulbs for conversion from electric power to visible light is about 15 lm/W, which is lower than that of fluorescent lamps (conversion efficiency, 90 lm/W), and therefore they impose larger environmental loads.
Patent document 1 suggests use of tantalum carbide having a higher melting point than that of tungsten for the filament. Patent document 1 discloses a method for producing a sintered body of a carbon compound containing tantalum carbide, which comprises mixing impalpable powder TaC, powdery carbide of Zr, Hf, or the like, and the like, molding the mixture, and heating the molded body at a temperature of 1600° C. or higher.
Patent document 2 discloses a method for manufacturing a coil-shaped tantalum carbide electrode. In this manufacturing method, tantalum is first processed into a coil shape, this coil is subjected to a heat treatment to remove the surface oxide film, and after a carbon source is introduced, the coil is further subjected to a heat treatment. Carbon is thereby made to permeate into the tantalum from the surface to form a coil-shaped electrode fully consisting of tantalum carbide or consisting of tantalum carbide and tantalum.
Patent document 3 discloses that if a TaC film is formed on a surface of a tungsten filament by an ion-plating method, superior heat resistance and stable thermionic or field emission current can be obtained.